Currently, soft magnetic materials are applied to various systems and devices, such as rotating electric machines (for example, motors and generators), potential transformers, inductors, transformers, magnetic inks and antenna devices, and thus, soft magnetic materials are regarded as very important materials. In these component parts, the real part of the magnetic permeability (real part of the relative magnetic permeability), μ′, of a soft magnetic material is utilized. Therefore, in the case of actual use, it is preferable to control μ′ in accordance with the working frequency band. Furthermore, in order to realize a highly efficient system, it is preferable to use a material having a loss that is as low as possible. That is, it is preferable that the imaginary part of the magnetic permeability (imaginary part of the relative magnetic permeability), μ″ (corresponding to a loss), is minimized as far as possible. In regard to the loss, the loss factor, tan δ(=μ″/μ′×100(%)), serves as a criterion, and as μ″ becomes smaller relative to μ′, the loss factor tan δ becomes smaller, which is preferable. In order to attain such conditions, it is preferable to make the core loss for the conditions of actual operation small, that is to say, it is preferable to make the eddy current loss, hysteresis loss, ferromagnetic resonance loss, and residual loss (other losses) as small as possible. In order to make the eddy current loss small, it is effective to increase the electrical resistance, or decrease the sizes of metal parts, or finely divide the magnetic domain structure. In order to make the hysteresis loss small, it is effective to reduce coercivity or increase the saturation magnetization. In order to make the ferromagnetic resonance loss small, it is effective to make the ferromagnetic resonance frequency higher by increasing the anisotropic magnetic field of the material. Furthermore, in recent years, since there is an increasing demand for handling of high electric power, it is required that losses be small, particularly under the operation conditions in which the effective magnetic field applied to the material is large, such as high current and high voltage. To attain this end, it is preferable that the saturation magnetization of a soft magnetic material is as large as possible so as not to cause magnetic saturation. Furthermore, in recent years, since size reduction of equipment is enabled by frequency increment, increase of the working frequency band that is utilized in systems and device equipment is underway, and there is an urgent need for the development of a magnetic material having high magnetic permeability and low losses at high frequency and having excellent characteristics.
Furthermore, in recent years, due to the heightened awareness of the issues on energy saving and environmental issues, there is a demand to increase the efficiency of systems as high as possible. Particularly, since motor systems are responsible for the greater portion of electric power consumption in the world, efficiency enhancement of motors is very important. Above all, a core and the like that constitute a motor are formed from soft magnetic materials, and it is requested to increase the magnetic permeability or saturation magnetization of soft magnetic materials as high as possible, or to make the losses as low as possible. Furthermore, in regard to the magnetic wedge that is used in some motors, there is a demand for minimizing losses as far as possible. There is the same demand also for systems using transformers. In motors, transformers and the like, the demand for size reduction is also high, along with efficiency enhancement. In order to realize size reduction, it is essential to maximize the magnetic permeability and saturation magnetization of the soft magnetic material as far as possible. Furthermore, in order to also prevent magnetic saturation, it is important to make saturation magnetization as high as possible. Moreover, the need for increasing the operation frequency of systems is also high, and thus, there is a demand to develop a material having low losses in high frequency bands.
Soft magnetic materials having high magnetic permeability and low losses are also used in inductance elements, antenna devices and the like, and among them, in recent years, attention has been paid to the application of soft magnetic materials particularly in power inductance elements that are used in power semiconductor devices. In recent years, the importance of energy saving and environmental protection has been actively advocated, and there have been demands for a reduction of the amount of CO2 emission and reduction of the dependency on fossil fuels. As the result, development of electric cars or hybrid cars that substitute gasoline cars is in active progress. Furthermore, technologies for utilizing natural energy such as solar power generation and wind power generation are regarded as key technologies for an energy saving society, and many developed countries are actively pushing ahead with the development of technologies for utilizing natural energy. Furthermore, the importance of establishment of home energy management systems (HEMS) and building and energy management systems (BEMS), which control the electric power generated by solar power generation, wind power generation or the like by a smart grid and supply the electric power to homes, offices and plants at high efficiency, as environment-friendly power saving system, has been actively advocated. In such a movement of energy saving, power semiconductor devices play a key role. Power semiconductor devices are semiconductor devices that control high electric power or energy with high efficiency, and examples thereof include discrete power semiconductor devices such as an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET), a power bipolar transistor and a power diode; power supply circuits such as a linear regulator and a switching regulator; and a large-scale integration (LSI) logic circuit for power management to control the above-mentioned devices. Power semiconductor devices are widely used in all sorts of equipment including electrical appliances, computers, automobiles and railways, and since expansion of the supply of these applied apparatuses, and an increase of the mounting ratio of power semiconductor devices in these apparatuses can be expected, a rapid growth in the market for power semiconductor devices in the future is anticipated. For example, inverters that are installed in many electrical appliances use power semiconductor devices nearly in all parts, and thereby extensive energy saving is made possible. Currently, silicon (Si) occupies a major part in power semiconductor devices; however, for a further increase in efficiency or further size reduction of equipment, utilizing silicon carbide (SiC) and gallium nitride (GaN) is considered effective. SiC and GaN have larger band gaps and larger breakdown fields than Si, and since the withstand voltage can be made higher, elements can be made thinner. Therefore, the on-resistance of semiconductor devices can be lowered, and it is effective for loss reduction and efficiency enhancement. Furthermore, since SiC or GaN has high carrier mobility, the switching frequency can be made higher, and this is effective for size reduction of elements. Furthermore, since SiC in particular has higher thermal conductivity than Si, the heat dissipation ability is higher, and operation at high temperature is enabled. Thus, cooling systems can be simplified, and this is effective for size reduction. From the viewpoints described above, development of SiC and GaN power semiconductor devices is actively in progress. However, in order to realize the development, development of power inductor elements that are used together with power semiconductor devices, that is, development of soft magnetic materials having high magnetic permeability (high magnetic permeability and low losses), is indispensable. In this case, regarding the characteristics required from magnetic materials, high magnetic permeability in the driving frequency bands, low magnetic loss, and high saturation magnetization that can cope with large current, are preferred. In a case in which saturation magnetization is high, it is difficult to cause magnetic saturation even if a high magnetic field is applied, and a decrease in the effective inductance value can be suppressed. As a result, the direct current superimposition characteristics of the device are improved, and the efficiency of the system is increased.
Furthermore, a magnetic material having high magnetic permeability and low losses at high frequency is expected to be applied to high frequency communication equipment devices such as antenna devices. As a method effective for size reduction of antennas and power saving, there is available a method of using an insulated substrate having high magnetic permeability (high magnetic permeability and low losses) as an antenna substrate, and performing transmission and reception of electric waves by dragging the electric waves that should reach an electronic component or a substrate inside a communication apparatus from antennas into the antenna substrate, without allowing the electric waves to reach the electronic component or substrate. As a result, size reduction of antennas and power saving are made possible, and at the same time, the resonance frequency band of the antennas can also be broadened, which is preferable.
In addition, examples of other characteristics such as high thermal stability, high strength, and high toughness are required when magnetic materials are incorporated into the various systems and devices described above. Also, in order for the magnetic materials to be applied to complicated shapes, a pressed powder is preferable to materials having a sheet shape or a ribbon shape. However, generally, in the case of the pressed powder, it is well known that characteristics such as saturation magnetization, magnetic permeability, losses, strength and toughness are not so good. Thus, enhancement of characteristics is preferable.
Next, in regard to conventional soft magnetic materials, the kinds of the soft magnetic materials and their problems will be described.
An example of an existing soft magnetic material for systems of 10 kH or less is a silicon steel sheet (FeSi). A silicon steel sheet is a material that is employed in most of rotating electric machines that have been used for a long time and handle large power, and the core materials of transformers. Highly characterized materials ranging from non-oriented silicon steel sheets to grain-oriented silicon steel sheets can be obtained, and compared to the early stage of discovery, a progress has been made; however, in recent years, it is considered that characteristics improvement has reached a limit. Regarding the characteristics, it is particularly critical to simultaneously satisfy high saturation magnetization, high magnetic permeability, and low losses. Studies on materials that surpass silicon steel sheets are actively conducted globally, mainly based on the compositions of amorphous materials and nanocrystalline materials; however, a material composition that surpasses silicon steel sheets in all aspects has not yet been found. Furthermore, studies also have been conducted on pressed powders that are applicable to complicated shapes; however, pressed powders have a defect that they have poor characteristics compared to sheets or ribbons.
Examples of conventional soft magnetic materials for systems of 10 kHz to 100 kHz include SENDUST (Fe—Si—Al), nanocrystalline FINEMET (Fe—Si—B—Cu—Nb), ribbons or pressed powders of Fe-based or Co-based amorphous glass, and MnZn-based ferrite materials. However, all of these materials do not completely satisfy characteristics such as high magnetic permeability, low losses, high saturation magnetization, high thermal stability, high strength and high toughness, and are insufficient.
Examples of conventional soft magnetic materials of 100 kHz or higher (MHz frequency band or higher) include NiZn-based ferrites and hexagonal ferrites; however, these materials have insufficient magnetic characteristics at high frequency.
From the circumstances described above, development of a magnetic material which has high saturation magnetization, high magnetic permeability, low losses, high thermal stability and excellent mechanical characteristics is preferable.